1. Field of the Invention
The present invention relates to tubulars for deep oil and gas wells and a process for the preparation of such tubulars. More particularly, the invention relates to tubulars, commonly known as Oil Country Tubular Goods (OCTG), for use in wells 15,000 to 35,000 feet deep, which may be subjected to high pressures, wide temperature ranges, and/or corrosive environments which may include hydrogen sulfide, carbon dioxide, and brine water along with hydrocarbons as constituents.
2. Discussion of the Prior Art
In recent years, work has been done to develop well tubulars having higher strength and better resistance to failure under severe stress and corrosive applications. This work was necessitated by the demand for tubulars suitable for use in deep wells in the range of 15,000 to 35,000 feet deep, where pressures and temperatures may exceed 15,000 psi and 250.degree. F., respectively. In addition, the tubulars may be subjected to highly corrosive atmospheres containing large quantities of hydrogen sulfide (H.sub.2 S), carbon dioxide (CO.sub.2), brine water, and/or associated hydrocarbons. Tubulars subjected to these conditions may fail in a matter of hours due to sulfide stress cracking.
The sulfide stress cracking characteristic of steel tubulars may be influenced by many factors, including the chemistry of the steel, the nature and amounts of alloying elements, the microstructure of the steel, the mechanical processing of the steel, and the nature of the heat treatment which may be provided.
Over the years, many attempts have been made to overcome the sulfide stress cracking problem in carbon steels, but prior to the present invention, no fully satisfactory solution has appeared.
The following patents illustrate the current state of the art.
A process for making seamless tubes using the so-called Pilger process, followed by reheating to forging temperatures (preferably in the neighborhood of 2100.degree. F.), and subsequent finishing in a plug mill, reeler, and sizing mill, is shown in U.S. Pat. No. 1,971,829.
U.S. Pat. Nos. 1,993,842, 2,275,801, and 2,361,318 disclose casing in which the collapse resistance is increased by subjecting the casing to cold radial compression up to 2 percent or slightly greater.
U.S. Pat. No. 2,184,624 discloses a heat treatment above the upper critical point followed by slow cooling prior to cold drawing to improve the machining qualities of a tube.
U.S. Pat. No. 2,293,938 suggests a combination of cold working a hot-rolled tube in the range of 5 to 10 percent, followed by a heat treatment below the lower critical point to increase the collapse resistance and maintain ductility.
Another method for improving properties, such as collapse resistance, is shown in U.S. Pat. No. 2,402,383, which discloses sizing a tubular casing formed about 3 to 10 percent over size while at a temperature somewhat below the lower critical temperature in the range of 650.degree. to 1000.degree. F.
U.S. Pat. No. 2,825,669 seeks to overcome sulfide stress corrosion cracking in a low carbon (less than 0.20C) composition by adding chromium and aluminum and heat treating in the range lying between Ac.sub.1 and Ac.sub.3 followed by an austenitizing heat treatment and an anneal. U.S. Pat. No. 2,825,669 also teaches that if the carbon is too high (e.g., above 0.20C), the resistance to stress corrosion cracking is impaired.
Another approach to the stress corrosion problem is low carbon steel (0.10 to 0.25C) by heat treating is disclosed in U.S. Pat. No. 2,895,861. In this patent, the steel is austenitized for about one hour, followed by air cooling. Thereafter, the steel is tempered above the Ac.sub.1 point for about one hour.
U.S. Pat. No. 3,655,465 discloses a two-stage heat treatment for oil well casing involving an intercritical heat treatment to produce not more than 50 percent of an austenite decomposition product upon cooling. Thereafter, the product is tempered below the lower critical point.
U.S. Pat. No. 3,992,231 shows still another approach to the problem of overcoming sulfide stress cracking in SAE 41XX steels. In this process, the steel is austenitized, quenched, and thereafter temper-stressed at a temperature below the transformation temperature by quenching the inner surface of the heated tube.
U.S. Pat. No. 4,032,368 discloses a process for reducing the time and energy required to perform an intercritical anneal for hypoeutectoid steel.
In U.S. Pat. No. 4,040,872, a method for strengthening a hypoeutectoid steel is disclosed. This comprises rapidly heating the steel into the austenite range (1350.degree. to 2000.degree. F.), quenching it, and then providing substantial cold working below the lower critical temperature.
Finally, in U.S. Pat. No. 4,226,645, a well casing having improved hydrogen sulfide stress cracking resistance is proposed. This patent discloses a tubular formed from an aluminum-killed steel containing controlled amounts of molybdenum, vanadium, and chromium, which is heat treated by austenitizing in the range of 1550.degree. to 1700.degree. F., quenching, and then tempering at 1200.degree. to 1400.degree. F. to produce a maximum hardness of 35 Rockwell C.
Specifications for deep well tubulars have been prepared by the American Petroleum Institute and various users. Such specifications describe grades of tubulars having yield strengths of, for example, 80,000, 90,000, 95,000, 110,000, 125,000, and 140,000 psi. A typical chemical composition for a modified 41XX steel for a 90,000 psi grade is specified in Table I, below:
TABLE I ______________________________________ Constituent Min. % Max. % ______________________________________ Carbon .20 .35 Manganese .35 .90 Chromium .80 1.50 Molybdenum .15 .75 Nickel -- .25 Copper -- .35 Phosphorus -- .04 Sulfur -- .04 Silicon -- .35 ______________________________________
The steel is fully killed and has a grain size of ASTM 5 or finer. The specification provides for an inside-outside quench following an austenitizing treatment so as to result in at least 90 percent martensite in the as-quenched condition. After tempering, the final hardness is specified in the range of 18 through 25 Rockwell C. Any surface defects, such as inclusions, laps, seams, tears, or blow holes, are required to be removed by grinding or machining to provide a minimum wall thickness of at least 87.5 percent of the nominal wall thickness.